1. Field of the Invention
The present invention relates to an index error estimating apparatus, an index error calibrating apparatus, and an index error estimating method.
2. Description of Related Art
A rotary encoder is an index error calibrating apparatus that detects a rotation angle of a rotation mechanism. The rotary encoder includes, as its basic structure, a circular grating disk and a detector, for example, the grating disk being etched with an index pattern including index marks of several hundreds to several hundred thousands in a radial direction in an external peripheral portion, the detector being arrayed on the grating disk and counting index marks which pass by as the grating disk rotates. Such a rotary encoder is provided such that the grating disk engages a rotating portion of an object to be measured. The number of index marks passing by the detector in association with rotation of the object to be measured is counted to detect a rotation angle of the object to be measured (i.e., rotation angle of the grating disk).
Normally, such a grating disk as described above has index marks that are artificially marked. Therefore, the index marks are not marked at even angular distances, and therefore deviate from ideal index mark positions (having even angular distances). Accordingly, detected values detected by the detector based on the index marks as described above contain errors (hereinafter referred to as index error). For calibrating such an index error, an equal division averaging method is conventionally used (see, for example, WATANABE, et al., “Automatic High Precision Calibration System for Rotary Encoder (First Issue),” Journal of the Japan Society for Precision Engineering Vol. 67 No. 7 (2001), 1091-1095).
FIG. 4 illustrates a conventional method to calibrate index errors. A conventional index error calibrating apparatus 100 employs, as shown in FIG. 4, a grating disk 200A for calibration provided on a coaxial rotation shaft 220; and a grating disk 200B for reference. An external periphery of the grating disk 200A for calibration is provided with a first detector 300A opposing the index pattern 210 of the grating disk 200A. An external periphery of the grating disk 200B for reference is provided with a plurality (six in the example shown in FIG. 4) of second detectors 300B opposing the index pattern 210 of the grating disk 200B, each of the detectors having an even angular distance along the circumferential direction of the grating disk 200B. A calculating apparatus (not shown in the drawing) configuring the index error calibrating apparatus 100 calculates differences between a detected value of the first detector 300A and each of the detected values of the second detectors 300B; obtains an index error by averaging the calculated differences; and calibrates the detected value of the first detector 300A with the index error.
Incidentally, the even division averaging method generally has a disadvantage where it is impossible to identify a Fourier component of an index error when the Fourier component is multiple of the number of detectors. Accordingly, in order to estimate a highly accurate index error that does not lack a component including a higher degree component, the number of the detectors (second detectors 300B) arranged at even angular distances needs to be increased. However, due to size restriction of the grating disk, the number of the detectors that can be arranged is also limited, thereby making it difficult to identify a higher degree component in practical use.